1. Mr. Vedprakash Maralapalle, Asst. Professor
Compass Traversing
Mr. Vedprakash Maralapalle, Asst. Professor
Department: B.E. Civil Engineering
Subject: Surveying – I
Semester: III
Teaching Aids Service by KRRC Information Section

2. INTRODUCTION & PURPOSE
In chain surveying, the area to be surveyed is divided into a number of triangles.
This method is suitable for fairly level ground covering small areas.
But when the area is large, undulating and crowded with many details, triangulation is not possible. In such an area, the method of traversing is adopted.
In traversing, the framework consist of a connected lines. The lengths are measured by chain or tape and the directions identified by angle measuring instruments.
In one of the methods, the angle measuring instrument used is the compass, hence, the process is compass traversing.

3. Compass Surveying is recommended when:
A large area to be surveyed.
The course of a river or coast line is to be surveyed.
The area is crowded with many details & triangulation is not possible.
Compass surveying is not recommended for area where local attraction is suspended due to the presence of magnetic substances like steel structures, iron ore deposits, electric cables conveying current, and so on.

4. CROSS STAFF
The cross staff is simple instrument for setting out right angles. There are three types of cross-staves.
Open
French

5. Open cross staff
The simplest form of cross staff is the open cross staff. It consists of two parts
(1) the head and
(2) the leg.
The head consists of four metal arms with vertical slits. The two pairs of arms are at right angles to each other. The vertical slits are meant for sighting the object and the ranging rods. The cross staff is mounted on a pole of length 1.5m and 2.5cm diameter. The pole is fitted with an iron shoe.

6. Open cross staff

7. French cross staff

8. OPTICAL SQUARE
It is more accurate than the cross staff and it can be used for locating objects situated at larger distances.
It is small and compact hand instrument and works on the principle of reflection (the angle between the first incident ray and the last reflected ray is twice the angle between the mirrors), in this case the angle between the mirrors is fixed at 45°.
So, the angle between the horizon sight and index sight will be 90°.
the observer should stand on the chain line and approximately at the position where the perpendicular is to be set up.
The optical square is held by the arm at the eye level. The ranging rod at the forward station B is observed thorough the unslivered portion on the lower part of the horizon glass.
Then the observer looks through the upper silvered portion of the horizon glass to see the image of the object P.
Suppose the observer find that the ranging rod B and image of object P do not conside. Then he should move forward or backward along the chain line until ranging rod B and image of P exactly coincide .
At this position the observer marks a point on the ground to locate the foot of the perpendicular.

10. OPTICAL SQUARE

11. True meridian:
The line or plane passing through the geographical north pole, geographical south pole and any point on the surface of the earth, is known as the true meridian .

12. Magnetic Meridian
When a magnetic needle is suspended freely and balanced properly, unaffected by magnetic substances, it indicates a direction. This direction is known as the magnetic meridian.
The angle of a line makes with the magnetic meridian, it is known as the magnetic bearing of line.

13. Arbitrary meridian: Sometimes for the survey of a small area, a convenient direction is assumed as a meridian, known as the arbitrary meridian. Grid Meridian- sometimes for preparing a map some state agencies assume several lines parallel to the true meridian for particular zone. Designation of magnetic bearing: Magnetic bearing are designated by two system: a. Whole circle bearing (WCB). b. Quadrantal bearing (QB).

14. Terms related to Chain Surveying
Survey Stations
Survey stations are the points at the beginning and at the end of a chain line they may also occur at any convenient position on the chain line. Such station may be
Main Stations
Subsidiary Stations
Tie Stations

15. Terms related to Chain Surveying

16. Terms related to Chain Surveying
Main Station Stations along the boundary of an area as controlling points are known as ‘Main Stations’ The lines joining the main station are called ‘ Main Survey Lines’. The main survey lines should cover the whole area to be surveyed. The main stations are denoted by Δ.

17. Terms related to Chain Surveying
Subsidiary Stations: Stations which are on the main survey lines or any other survey lines are known as ‘ Subsidiary Stations’ these stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details.

18. Terms related to Chain Surveying
Tie Stations:
These stations are also subsidiary stations taken on the main survey lines. Lines joining the stations are known as ‘ Tie lines’ Tie lines are taken to locate interior details.

19. Terms related to Chain Surveying
Main Survey Lines:
The line joining the main stations are called main survey lines or chain lines.

20. Terms related to Chain Surveying
Base Line: The line on which the framework of the survey is built is known as ‘ Base line’. It is the most important line of the survey. Generally the longest of the main survey lines is considered as the base line. This lines should be taken through fairly level ground, and should be measured very carefully and accurately.

21. Terms related to Chain Surveying
Check Line
The line joining the apex point of a triangle to some fixed points on its base is known as ‘ Check line’. It is taken to check the accuracy of the triangle. Sometimes this line helps to locate interior details.

22. Terms related to Chain Surveying
Tie Line
A line joining tie stations is termed as a tie line. It is run to take the interior details which are far away from the main lines and also to avoids long offsets. It can also serve as check line.

23. Selection of Survey Station
The following points should be considered while selecting survey stations:
It should be visible from at least two or more stations.
As far as possible main lines should run on level ground.
All triangles should be well conditioned (No angle less than 30º).
Main network should have as few lines as possible.

24. Selection of Survey Station
Each main triangle should have at least one check line.
Obstacles to ranging and chaining should be avoided.
Sides of the larger triangles should pass as close to boundary lines as possible.
Trespassing and frequent crossing of the roads should be avoided

25. Optical Square and Prism Square

26. Operation in Chain Surveying
The following operations are involved in chain surveying.
Chaining
Ranging
Offsetting
These three operations are done simultaneously during chain Surveying.

27. Perpendicular Offsets Using Optical Square and Prism Square

28. Perpendicular Offsets Using Optical Square and Prism Square

30. Perpendicular Offsets Using Optical Square and Prism Square

31. Perpendicular Offsets Using Optical Square and Prism Square

32. Perpendicular Offsets Using Optical Square and Prism Square

33. Whole circle bearing (WCB)
The magnetic bearing of a line measured clockwise from the north pole towards the line, is known as the whole circle bearing.
WCB of OA = 60º
WCB of OB = 140º
WCB of OC = 210º
WCB of OD = 320º

34. Quadrantal bearing (QB)
The magnetic bearing of a line measured clockwise or counterclockwise from the north pole or south pole (whichever is nearer the line) towards east or west, is known as quadrantal bearing of the line.
QB of OA = N 60º E
QB of OB = S 40º E
QB of OC = S 30º W
QB of OD = N 40º W

35. Reduced bearing (RB): when the whole circle bearing of a line is converted to quadrantal bearing , it is termed reduced bearing.

36. Fore and back bearing: The bearing of a line measured in the direction of the progress of survey is called the fore bearing (FB) of the line.
The bearing of a line measured in the direction opposite to the survey is called the back bearing (BB) of the line.

37. Convert the following whole circle bearing quadrant or reduced bearings :
( i ) 42 30 ( ii ) 126 15
( iii ) 242 45 ( iv ) 328 10
Solution
( i ) W.C.B. = 42 30,Quadrant bearing
= N 42 30E
( ii ) W.C.B = 126 15, Reduced bearing or R.B.
= 180 - W.C.B. = 180 - 126 15
= S 53 45 E
( iii ) W.C.B. = 242 45 , R.B. = W.C.B. – 180o
= 242 45 - 180 = 62 45
= S 62 45 W
( iv ) W.C.B. = 328 10 , R.B. = 360o – W.C.B.
= 360o - 328 10
= N 31o 50 W

38. Solution ( I ) N 65 12 E ( ii ) S 3648 E
Convert the following reduced bearings to whole circle bearings:
( I ) N 65 12 E ( ii ) S 3648 E
( iii ) S 38 18 W ( iv ) N 26 32W
Solution
( I ) R.B. = N 65 12 E , W.C.B. = R.B. = 65 12
( ii ) R.B = S 36 48 E , W.C.B. = 180 - R.B.
= 180 - 36 48 =143 12
( iii ) R.B. = S 138 18 W , W.C.B. = 180 + R.B.
= 180 + 38 18 = 218 18
( iv ) R.B. = N 26 32 W , W.C.B = 360 - R.B.
= 360  = 333 28

39. Local Attraction
A magnetic needle indicates the north direction when freely suspended or pivoted.
But if the needle comes near some magnetic substances, such as iron ore, steel structure, electric cable conveying current; etc.
It is found to be deflected from its true direction, & does not show the actual north.
This disturbing influence of magnetic substances is known as local attraction.
If the difference of fore & back bearing of the line is exactly 180°, then there is no local attraction.

40. Traversing
Surveying which involves series of connected lines is known as traversing. The sides of the traverse are known as traverse legs.
Closed
Open

41. Closed Traverse
When a series of connected lines forms a closed circuit , i.e.
when the finishing point coincide with the starting point of a survey, it is called a Closed Traverse.
Here ABCDEA represents a closed traverse.
This is suitable for the survey of boundaries of ponds, forests, estates, etc.

42. Open Traverse
When a sequence of connected lines extends along a general direction & does not return to the starting point , it is called a Open Traverse.
Here ABCDE represents a open traverse.
This is suitable for the survey of roads, rivers, coast lines, etc.

43. Types of compass prismatic compass Surveyor’s compass
There are two types of compass: -
prismatic compass
Surveyor’s compass

45. Prismatic compass
In this compass, the readings are taken with the help of a prism.

46. Compass box : The compass box is a circular metallic box (the metal should be non magnetic) of dia. 8 to 10 cm.
Magnetic needle & Graduated Ring :
Prismatic Compass comprises of a magnetic needle attached to the circular ring made up of aluminum.
The needle is on the pivot and will orient itself in the magnetic meridian Therefore the north and south ends of the ring will be in this direction.
The ring is graduated from 0° to 360° clockwise, & the graduations begin from the south end of needle. Thus 0° is marked at the south.

47. Sight vane & Prism : The Sight vane & reflecting prism are fixed diametrically opposite to the box. The Sight vane is hinged with the metal box & consists of a horsehair at the centre. A triangular prism is fitted at the eye sight. Dark Glasses : 2 dark glasses are provided with the prism. Adjustable Mirror : A mirror is provided with the sight vane. The mirror can be lowered or raised, and can also be inclined. If any object is too low or too high with respect to the line of sight.

48. Brake pin : A brake pin is provided just at the base of the sight vane
Brake pin : A brake pin is provided just at the base of the sight vane. If pressed gently, it stops the oscillations of the ring. Lifting pin : A lifting pin is provided just below the sight vane. When the sight vane is folded, it presses the lifting pin. Lifting pin then lifts the magnetic needle out of the pivot point to prevent damage to the pivot head. Glass Cover : A glass cover is provided on top of the box to protect the aluminum ring from dust.

49. Surveyor’s compass

50. Sr. No. Surveyors compass Prismatic compass 1
The needle is of edge bar type and also acts as an Index
The needle is broad needle type but does not act as an Index 2
The graduated ring is attached to the box and not to the needle.
The graduated ring is attached with needle 3
The graduation are in QB system having 0 deg. To 90 deg
The graduation are in WCB system having o degree to 360 degree 4
The reading are taken directly be seeing through the top of the glass.
The readings are taken with the help of prism provided at the eye slit. 5
Sighting and reading can not be done simultaneously from one position of the observer.
Sighting and reading can be done simultaneously from one position of the observer. 6
The instrument can not be used without tripod
The instrument can be used without tripod

51. Temporary adjustment of prismatic compass
Fixing the compass with tripod stand
Centering
Levelling
Adjustment of prism
Observation of bearings

52. Conventional signs

57. Sources of Error in compass
The errors may be classified as  
Instrumental errors
Personal errors
Errors due to natural causes

58. Instrumental errors : The needle not being perfectly straight.
Pivot being bent
Improper balancing weight
Plane of sight not being vertical
Line of sight not passing through the center of graduated ring
Sight vane may not be vertical.
The horse hair may not be straight & vertical.
The graduations of the ring may not be uniform.

59. Personal errors They may be due to the following reasons:
In accurate leveling of the compass box.
In accurate centering.
In accurate bisection of objects.
Carelessness in reading and recording. 
The observer may be carrying magnetic substances.

60. Natural Errors: They may be due to following reasons:
Variation in declination
Local attraction due to proximity of local attraction forces.
Magnetic changes in the atmosphere due to clouds and storms.

61. Precautions to taken in Compass Surveying.
Centering should be done perfectly.
Stop the vibrations of the needle before taking observations by gently pressing the knob .
Set up the compass and level properly before taking the observations.
Take double readings at each important station .
Take readings in the proper direction , i.e. clockwise and in the increasing order .
Avoid keeping magnetic materials in case proximity of the station of observation .
There should be no iron objects , such as keys , buttons , etc on the person who is taking the reading .
The cover should not be dusted with a handkerchief, because the glass may be charged with electricity & the needle may be deflect from its true direction. The glass cover should be cleaned with a moist finger.

62. References
Surveying and Levelling: Vol-I and II: Kanetkar and Kulkarni, Pune Vidyarthi Griha, Pune.
Surveying and Levelling: N N Basak, Tata McGraw Hill, New Delhi.
Surveying: R. Agor, Khanna Publishers.
Surveying: Vol-I: Dr K.R. Arora, Standard Book House.
Kanpur and IIT Madras.
Google images.